1. Field of the Invention
The present invention generally relates to a machine tool and a tool holder. More particularly, the present invention relates to a machine tool having a turning function using a fixed tool and a milling function using a rotary tool, and a tool holder employed in such a machine tool.
2. Description of the Background Art
In the field of conventional machine tools, Japanese Utility Model Laying-Open No. 62-104833, for example, discloses a positioning device for a tool bit holder in a machine tool directed to setting the position of a cutting tool accurately (Patent Document 1).
As a positioning device for a tool bit holder disclosed in Patent Document 1, a spindle head of a turning center, for example, is envisaged. The spindle head includes a support sleeve, and a spindle supported rotatably in the support sleeve. The spindle is hollow. A main chuck for drawing in a main engagement section of a tool bit holder is mounted in a retractable manner at the center hole of the spindle. Four holes are formed in the support sleeve. An auxiliary chuck having a configuration similar to that of the main chuck for drawing in an auxiliary engagement section of the tool bit holder is loaded in each hole.
As an automatic tool changer (ATC) mounted on a machine tool, there is known a device for exchanging a tool holder loaded to the spindle for another tool holder, using a double arm having a gripper provided at either end, i.e. the so-called double arm scheme. An automatic tool changer of the double arm scheme can complete the tool exchange operation in a short period of time since the tool holder loaded to the spindle and another tool holder in a standby state are replaced simultaneously by a series of rotating and sliding movements of the double arm.
In the positioning device for a tool bit holder disclosed in the aforementioned Patent Document 1, the requirement of firmly coupling the tool bit holder with the spindle head to withstand a great cutting force is satisfied, not only by the coupling of the spindle with the tool bit holder by means of the main engagement section and main chuck, but also by the coupling of the support sleeve with the tool bit holder by means of a plurality of auxiliary engagement sections and auxiliary chucks.
In the positioning device for a tool bit holder disclosed in Patent Document 1, the auxiliary engagement sections are provided at the four corners of a tool bit holder formed in the shape of a square. An auxiliary chuck is provided at a position corresponding to a relevant auxiliary engagement section. In the case where an automatic tool changer of the double arm scheme is applied to a machine tool including a positioning device of such a tool bit holder, the double arm that moves over the end face of the support sleeve in association with the rotating motion will interfere with the coupling section between the auxiliary engagement section and auxiliary chuck during the tool exchange operation.